Low Frequency Magentic Shielding: An

Low Frequency Magentic Shielding: An Integrated Solution
Author : Rich Emrich and Andrew Wang
05/21/2009
Innovative direct coating technique as an alternative to conventional foils.
Rich Emrich and Andrew Wang, Integran Technologies Inc.; Toronto, Canada
INTRODUCTION
Many electrically sensitive devices, such as transducers, sensors, detectors, and test
instrumentation require protection from electromagnetic interference (EMI). Higher frequency
interference is normally shielded using a thin conductive metallization layer. Unfortunately,
simple electrically conductive layers (e.g., copper, aluminum) are transparent to low frequency
magnetic fields that can cause noise in an electronic device. This low frequency magnetic
interference can be emitted from sources such as switches, motors, power supplies, and
transformers and is typically a challenging EMI shielding problem. If the accuracy or precision
of the underlying circuitry is critical, low frequency magnetic shielding is required and is
normally achieved through the use of specialty ferromagnetic metal alloys with high magnetic
permeability (Figure 1). Materials with high magnetic permeability protect sensitive devices
from the electrical noise caused by magnetic fields by redirecting the magnetic field through the
shielding material and away from the protected device. In the same fashion, devices that emit
low frequency magnetic fields can also be isolated by using materials with high magnetic
permeability.
Figure 1. Magnetic shielding ability at different frequencies. Adapted from: Ott, H.W., Noise Reduction Techniques in
Electronic Systems (New York: John Wiley & Sons, © 1976)
Aside from the permeability, the strength (or flux density) of a magnetic shield must be
considered as well. Each material has a magnetic saturation, which determines the magnetic
field strength that can be effectively shielded. In strong fields, a magnetic shield can become
―full,‖ after which it will no longer provide effective shielding. Many materials chosen for high
magnetic intensity applications have good saturation, but higher saturation often comes with a
sacrifice in terms of the absolute permeability that can be achieved. For the purpose of this
article, we will focus on applications that are permeability limited, not saturation limited.
REVIEW OF SOLUTIONS
Shielding Materials: Magnetic Properties and Grain Structure
The most common solution for shielding low frequency EMI is to use a high permeability
magnetic shielding sheet metal or foil. Typical examples of shielding foils include specialty
ferromagnetic alloys branded as MuMetal®, Netic®, Finemet® and Metglas®. In many of
these alloy foils, the magnetic shielding properties rely on maintaining a large crystalline grain
size of around 100 μm or more in the material. Since the materials have an equilibrium state
grain size of around 1–10 μm, the large grain structure must be achieved through annealing the
metal at high temperatures, often in a tightly controlled atmosphere to control impurities. Other
foils have an amorphous, i.e., no crystalline grains, or nano-crystalline structure that is normally
achieved through complex manufacturing techniques including rapid solidification and high
frequency annealing processes.
Existing Shielding Solutions
Although specialty foils are very effective EMI shields, they are best suited to shielding simply
shaped parts. There are currently a few methods for creating parts with relatively complex
geometries, described in Table 1.
Table 1. Existing shield solutions.
Foil Lay-Up and Formed Foils
Since sheet metal or foils are manufactured flat, materials that resist forming operations, like
glassy amorphous alloys, must be cut to shape and must be laid onto flat surfaces to achieve the
necessary part shielding. Although these materials have excellent shielding performance, this
lay-up approach is a cumbersome method for shielding parts with any contour. Other less brittle
shielding materials can also be cut and formed to the shape required. For example, a sensitive
transducer measuring a faint analog current might require a ―can‖ of material over the device
that will be integrated into the enclosure or onto the circuit board. This shape would need to be
cut and stamped from the original sheet metal or foil. Although this process adds cost to the
shielding process, it is not, in itself, a particularly challenging operation.
Unfortunately, the forming operation reduces the effectiveness of the shield by introducing
deformation into the material—destroying the key material characteristic that generates the high
permeability. To restore the defect-free structure, the material must be annealed once more in a
high temperature controlled environment to restore shielding effectiveness. This necessity
presents a restriction for customers as the forming operation now has to be coupled with an
annealing operation, limiting the possible manufacturers, or adding an additional step in the
supply chain.
Apart from shielding efficacy, the forming step also sets a practical size limit as complex or small
geometries are difficult to produce. As electronic structures and packaging shrinks, discrete,
formed shields may not be a viable alternative. Lastly, once the discrete shield is cut, formed,
and annealed, it still needs to be integrated into the assembly of the shielded part. This
integration requires labor and often relies on adhesives, further adding to the assembly cost and
complexity.
Figure 2. Part directly coated with high magnetic permeability metal.
Metal Injection Molding (MIM)
Fine specialty ferromagnetic powders can now be injection molded with a binder to create
complex parts that are effective low frequency magnetic shields. After molding, the parts are
thermally or chemically treated and then sintered at high temperatures to remove the binder.
Secondary operations are sometimes required to achieve the final shape. Although this process
can make effective shields in relatively complex forms, part shrinkage must be accounted for, as
well as mechanical considerations such as part porosity and brittleness. As tooling costs can be
high, this method is ultimately best suited for high volume but relatively small parts.
Metal Components
Conventional metal components, such as steel stampings and castings, can also be used for lowfrequency EMI shielding. Often, this can be a very cost-effective method, but the additional
weight can be problematic in some applications. Much thicker steel stock will be required to
achieve the same shielding effectiveness as specialty ferromagnetic alloys and in many cases a
minimum thickness of steel is required for the forming operations. In many applications where
weight is critical, steel is being replaced with other materials, such as polymers or aluminum
that have little or no low frequency shielding capability. These materials must often be
combined with one of the other shielding solutions.
ALTERNATIVE SOLUTION; DIRECT PART COATING
An alternative solution to Table 1 is to apply a high magnetic permeability metal coating directly
to the surface of a part (Figure 2). This process is easily adapted to small and complex shield
shapes and avoids many of the possible drawbacks of a discrete shield, including the elimination
of annealing steps. Coating parts with conductive surfaces is often used for high frequency EMI
shielding in electronics applications using techniques such as physical vapor deposition (PVD),
conductive paints, and electroless and electrolytic plating. In these cases, the primary
requirement is a thin conductive coating. With recent developments in nanocrystalline
ferromagnetic coatings, the same concept can now be used for low frequency shielding.
Figure 3. Trend of coercivity vs. grain size Source: Adapted from G. Herzer, Mat. Sci. Eng., A133 (1991).
Magnetic Properties of Nanostructed Coatings
In contrast to conventional shielding materials that derive magnetic properties from their large,
un-deformed grain structure, a ferromagnetic alloy coating with a very small (nanometer, in
fact) grain structure can also achieve high permeability—and therefore similar magnetic
shielding performance. Figure 3 illustrates the relationship between coercivity and grain size in
a ferromagnetic material. Typically magnetic shielding materials with low coercivity also possess
high permeability. Therefore, as shown in the graph in Figure 3, when the grain size of the
material is reduced to the nanometer scale, the coercivity is minimized and the permeability is
maximized. A similar effect also occurs with very large grain size.
Figure 4. Aluminum housing coated with nanocrystaline ferromagnetic coating.
Processing and Design Benefits and Limitations
Although the magnetic shielding characteristics of nanocrystalline ferromagnetic metal coatings
are good, the real benefit lies in the processing and design flexibility that this
material affords. This material is used most frequently as a coating deposited directly onto part
substrates such as metals (e.g., aluminum enclosures), polymers (e.g., thermoplastic formed
parts), and composites (e.g., carbon fiber epoxy structures)—a process that avoids any forming
operations. The coating can be integrated directly into the part packaging or enclosure, a design
option that reduces part count by eliminating the discrete formed shield and related adhesives
and labor. Selective coating is possible, allowing shielding performance that can be delivered
where it is needed most and avoiding unnecessary added weight. For injection molded polymers
in particular, monolithic part integration is now possible because the shielding function can be
molded directly into the larger electronics enclosure or part packaging.
Figure 5. Complex polymer housing coated with nanocrystaline metal shielding.
Aside from avoiding the forming operation, deformation of these nanocrystalline coatings does
not affect the grain size, thus retaining shielding effectiveness and avoiding both a loss in
performance and a re-annealing step. However, the long-term operating temperature must be
kept under a threshold of approximately 400°F to avoid grain growth back to a larger crystalline
equilibrium state. (The exact temperature will vary depending on the alloy composition.)
Figure 5. Polymeric cell phone component selectively coated with nanocrystalline metal shielding.
As with any solution, this approach has its own drawbacks. The metal coating operation is a
secondary operation that adds cost to the part. In addition, the thickness and distribution of
metal in a typical deposition
process for nanocrystalline metals is influenced by part geometry. Despite these factors, the
application process is industrially scalable, and the metal coating approach is effective for
applications in which discrete shields are awkward or impractical. An example of a metal part
that can benefit from direct part coating is shown in Figure 4 that depicts a complex aluminum
machined housing that required shielding. The application is weight sensitive and required
layups of MuMetal R foils and labor intensive work to bend the foils and to bond them to shield
the corners. As an alternative, a nanocrystalline ferromagnetic coating, 50 microns thick was
applied. The part has performed well in initial EMI testing, and the majority of the foils could be
eliminated, decreasing both weight and cost. The part also benefited from increased surface
hardness and an aesthetically pleasing finish.
Polymer parts can also benefit greatly from direct coating solutions (Figure 5). Complex
moldings may be coated, a step that also provides additional stiffness, strength, and surface
hardness to create multifunctional parts that may be lighter and thinner than die-cast parts.
Coating a complex polymer housing with a nanocrystalline metal will add stiffness and strength
to the polymer part. Figure 6 shows a polymer housing with the coating applied selectively. This
selective application delivers shielding performance where it is required and eliminates
unnecessary weight. Using a selective coating process can enable monolithic integration of
discrete parts in an assembly, reducing part cost.
Material property benefits
Another useful property of nanocrystalline coatings is that they have a higher strength and
hardness than coarse-grained equivalent material (Table 2). Applied to polymer substrates, the
increase in strength can be used to dramatically increase the strength and stiffness of the hybrid
part. A relatively weak or flexible polymer enclosure can now become a rigid, durable structural
part.The high yield strength and good ductility also make the coating particularly well suited for
polymer and composite applications that often undergo a lot of flexing. A lower strength coating
would plastically deform with flexion; whereas traditional high strength coatings are normally
brittle, a situation that typically leads to failure at low loads.
Table 2. Comparison of nanocrystalline coating vs. polycrystalline foil.
Nanocrystalline metals can now be made fully dense—i.e., they have no voids or porosity.
Having a highly impermeable coating could be beneficial either for keeping liquids or gases from
penetrating the surface of a part, as would be the case for protecting a polymer case from
chemical attack, or for keeping the part from outgassing into the environment—often a
secondary consideration in semiconductor or space applications.
CONCLUSION
While specialty ferromagnetic foils and sheet metal stock are useful for creating relatively simple
geometry discrete magnetic shields, they require forming, annealing, and assembly steps and
are not amenable to small or complex parts. Metal injection molding and standard steel parts
can also solve many shielding problems but can run into challenges in weight and lower
shielding effectiveness. Nanocrystalline ferromagnetic metal coatings offer an alternative
solution for directly shielding metal, polymer, or composite parts which can reduce cost by
simplifying the supply chain and assembly process.
This metal coating process also offers the possibility of creating small or complex shields that
were previously impractical with discrete shielding methods. Additional benefits can also be
realized by using the superior mechanical properties of nanocrystalline metals to strengthen,
stiffen, render impermeable, or add durability to a polymeric part.
ABOUT THE AUTHORS
Rich Emrich is VP of Business Development for Integran Technologies. Rich has an
undergraduate engineering degree from the University of Guelph, a Masters in Engineering
from the University of British Columbia and is a Project Management Professional. He has led
product development in a wide variety of industries including mass spectrometer design for
the pharmaceutical industry, optical networking component design, and radioactive drug
process engineering. In his current position, he is responsible for finding good magnetic
shielding applications for Integran’s Nanocrystalline Ferromagnetic “Nanovate™ EM”
coating.
Andrew Wang is VP of Product Development at Integran Technologies. Andrew has a
mechanical engineering background, receiving his undergraduate degree at The University of
Waterloo and his Master’s degree at The Massachusetts Institute of Technology, specializing in
Advanced Manufacturing. In his current role, Andrew assists customers with developing new
products using Integran’s novel Nanocrystalline metals.
Source: http://www.interferencetechnology.com/low-frequency-magentic-shielding-an-integratedsolution/